1. Field of the Invention
This invention relates generally to application of light emitting elements such as light emitting diodes and laser diodes to evaluation of optical devices, amplification of optical signals, and optical communication, and more specifically to a light source apparatus suitable for production of pump light for an optical amplifier or for evaluation of a passive optical device as well as an optical amplifier and an optical communication system which includes a light source apparatus of the type just mentioned.
2. Description of the Related Art
As a result of establishment of a technology for production of silica fibers of a low loss (for example, 0.2 dB/km), an optical communication system which includes an optical fiber as a transmission line has been put into practical use. The optical communication system includes a first terminal station having an optical transmitter which outputs an optical signal, an optical fiber transmission line for transmitting the optical signal, and a second terminal station having an optical receiver which receives the optical signal transmitted thereto by the optical fiber transmission line.
In order to compensate for a loss in an optical fiber transmission line, one or more optical repeaters each having an optical amplifier for amplifying an optical signal are interposed intermediately in the optical fiber transmission line. The optical amplifier includes an optical amplification medium to which an optical signal is supplied, and means for pumping (exciting) the optical amplification medium so that the optical amplification medium may have an amplification band which includes a wavelength of the optical signal. For example, an erbium doped fiber amplifier (EDFA) for amplifying an optical signal of the wavelength band of 1.55 xcexcm includes a doped fiber doped with erbium (Er) and having a first end and a second end, and a pump light source for supplying pump light to the doped fiber through at least one of the first and second ends. When an optical signal to be amplified is supplied to the doped fiber through the first end, then the optical signal is amplified in the doped fiber in accordance with the principle of stimulated emission, and the amplified optical signal is outputted through the second end.
Since an EDFA requires a pump light source for pumping a doped fiber in this manner, an electronic circuit for supplying power to the pump light source is provided additionally for the EDFA. On the other hand, where an optical fiber transmission line is laid on the bottom of the sea, since an optical repeater having the EDFA must be kept sunk on the bottom of the sea, maintenance of the optical repeater is very difficult. According, it is required for an optical fiber communication system which includes an EDFA that the maintenance be easy.
As a pump light source for an optical amplifier, a light emitting element such as a light emitting diode or a laser diode is used. In order to raise the optical power outputted from an optical amplifier, the maximum optical power of pump light to be supplied to a doped fiber should be raised. Since the output optical power of one light emitting element is limited, in order to obtain a higher optical power, two light emitting elements which output first pump light and second pump light having different wavelengths from each other are used and the first pump light and the second pump light are wavelength division multiplexed.
For example, in order to effect wavelength division multiplexing in a pump band of the 1.48 xcexcm wavelength band (1.45 xcexcm to 1.50 xcexcm) which is adopted by the EDFA, a spectrum width of approximately 20 nm is required for each of the first pump light and the second pump light, and the dispersion of the center wavelength must be restricted to approximately xc2x15 nm. However, the spectrum of a light emitting element is liable to be dispersed due to an error in production or the like, and there is a problem that the yield of light emitting elements which can be used for wavelength division multiplexing is low.
Incidentally, in order to evaluate a passive optical device represented by an optical multiplexer which is used for wavelength division multiplexing, a light source apparatus for obtaining a beam of light having a desired center wavelength is required. While a variable wavelength light source is known as one of light source apparatus of the type mentioned, the variable wavelength light source has a problem in that it is expensive. Further, while it may be proposed to apply a band-pass filter to a beam of light outputted from a light emitting diode having a comparatively broad spectrum to obtain a beam of light having a center wavelength necessary for measurement, since the beam of light obtained in this instance is reduced in power, the dynamic range in measurement is decreased.
It is to be noted that, as a technique which seems to pertain to the light source apparatus of the present application, an apparatus disclosed in Japanese Patent Laid-Open Application No. Heisei 7-45890 or Japanese Patent Laid-Open Application No. Showa 62-154685 is known. The former discloses an external resonator type semiconductor laser wherein an external resonator for a semiconductor laser is formed using a diffraction grating and driving current for the semiconductor laser is controlled in accordance with diffracted light from the diffraction grating. The latter discloses a light source for wavelength multiplex communication wherein a single common diffraction grating is applied to a plurality of semiconductor lasers.
Accordingly, it is an object of the present invention to provide a light source apparatus which can produce a beam of light having a desired center wavelength irrespective of a spectrum of a light emitting element.
It is another object of the present invention to provide a light source apparatus which can produce a beam of light having a maximum power higher than the maximum power of a beam of light outputted from a light emitting element.
It is a further object of the present invention to provide an optical amplifier which provides a high output optical power.
It is a still further object of the present invention to provide an optical fiber communication system wherein a loss in an optical fiber transmission line can be compensated for and the maintenance is easy.
According to an aspect of the present invention, there is provided a light source apparatus which includes a light emitting element having a gain band and a band-reflection filter optically connected to the light emitting element. The light emitting element outputs a light beam having a spectrum which is determined by the gain band. The band-reflection filter produces, from the light beam outputted from the light emitting element, a transmission beam and a reflection beam which returns to the light emitting element. The band-reflection filter has a reflection band included in the gain band of the light emitting element and narrower than the gain band, and the transmission beam of the band-reflection filter has a maximum power higher than the maximum power of the light beam to be outputted from the light emitting element.
With the construction, pump light having a maximum power higher than the maximum power of the light to be outputted from the light emitting element can be obtained, and one of the objects of the present invention is attained. Further, since the center wavelength of the light beam to be obtained is determined by the reflection band of the band reflection filter, even if the gain band or spectrum of the light emitting element has some dispersion, a light beam having a desired center frequency can be obtained as far as the reflection band is included in the range of the dispersion, and another one of the objects of the present invention is attained.
According to another aspect of the present invention, there is provided an optical amplifier which comprises an optical amplification medium to which a signal light beam is supplied, and means for pumping the optical amplification medium so that the optical amplification medium may have an amplification band including a wavelength of the signal light beam. The means for pumping includes a light emitting element having a gain band and a band-reflection filter optically connected to the light emitting element. The light emitting element outputs a light beam having a spectrum which is determined by the gain band. The band reflection filter produces, from the light beam outputted from the light emitting element, a transmission beam and a reflection beam which returns to the light emitting element. Then, the transmission beam is supplied to the optical amplification medium.
With the construction, as the reflection beam from the band-reflection filter returns to the light emitting element which has the gain band, the power of the reflection beam is increased, and pump light of a higher power can be obtained. Consequently, an optical amplifier having a high output optical power can be provided.
According to a further aspect of the present invention, there is provided an optical communication system which comprises a first terminal station including an optical transmitter for outputting an optical signal, an optical fiber transmission line for transmitting the optical signal, and a second terminal station including an optical receiver for receiving the optical signal transmitted by the optical fiber transmission line. The optical fiber transmission line includes, at least at a portion in the longitudinal direction thereof, a doped fiber doped with a rare earth element. At least one of the first and second terminal stations further includes means for pumping the doped fiber so that the doped fiber may have an amplification band which includes a wavelength of the optical signal. The means for pumping includes a light emitting element having a gain band and a band reflection filter optically connected to the light emitting element. The light emitting element outputs a light beam having a spectrum which is determined by the gain band. The band reflection filter produces, from the light beam outputted from the light emitting element, a transmission beam and a reflection beam which returns to the light emitting element. Then, the transmission beam is supplied to the doped fiber.
With the construction, since a light emitting element and an electronic circuit for driving the light emitting element can be provided in the first or second terminal station, maintenance is facilitated. Further, since the optical fiber transmission line includes a doped fiber, compensation for the loss in the optical fiber transmission line can be achieved.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the present invention.